Membrane-membrane and membrane-substrate adhesion during dissection of gap junctions with the atomic-force microscope

The gap junction is a specialized region of the plasma membrane that consists of an array of cell-to-cell ion channels. These channels form where the membranes from two cells come together, and the gap junction is therefore composed of two lipid bilayers. The atomic force microscope (AFM) can be used to dissect the gap junction, removing one membrane and exposing the extracellular domains of the second. The force required to dissect the membrane, near 10^-8 N vertical force for gap junctions adsorbed to mica, provides a measure of the strength of the interaction between the two membranes. Since a single membrane is left in contact with the mica, this interaction must be stronger than the membrane-membrane interaction. Non-junctional membrane attached to the gap junctions is easily removed with the AFM tip while the gap junction membrane remains attached to the mica, providing evidence that the interaction with the mica is mainly mediated by protein-mica interactions. Consistent with this hypothesis is the observation that material trapped under the membrane sometimes results in pieces of membrane above the material being pulled out during dissection. These results lay the foundation for examining the molecular details of the basis for membrane- membrane and membrane-substrate adhesion.